The optical properties of the Ga1-xZnxN1-xOx alloy (x=0.0, 0.25, 0.50, 0.75, and 1.00) are studied by first-principles means, employing the GW method to describe single-particle excitations and the Bethe-Salpeter equation (BSE) to model the two-particle exciton interactions. Intriguingly, we find that the band gaps of the Ga1-xZnxN1-xOx alloy are reduced significantly compared with that of bulk ZnO and GaN. By including the electron-hole interactions within the BSE approach, the imaginary part epsilon(2)(omega) of the dielectric function shows an optical absorption enhancement in the low energy region with the exciton peak below the band gap energy. By comparing the energy difference between the exciton absorption peaks E-ex and the energy gaps E-g, we qualitatively estimate that the strength of excitonic coupling is weaker in the Ga1-xZnxN1-xOx alloy than in both GaN and ZnO. Interestingly, the exciton absorption intensity increases with respect to ZnO content. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.